> I wonder why so many companies are happily paying Oracle [...] instead of sponsoring an open source project like PostgreSQL.

Corporate world is ruled by liability. When you are working on a multi-million dollar project and the database breaks, you want to be able to put as much responsibility on the 3-rd party as much as possible.

Open Source projects come with no guarantee. If someone hacks into a system because of a flaw in the Open Source project you are screwed. If it's a software delivered by Oracle, then Oracle is responsible and needs to pay your million dollar fine.

Of course I'm simplifying, because there's probably still a lot of legal process behind it depending on the country, but it's essentially that.